Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
In a membrane filtration process, the method used to physically clean membranes is of vital importance. An efficient membrane cleaning strategy can maintain a stable permeability of the membrane and reduce the frequency of chemical cleans. A commonly used method to physically clean membranes is a backwash (also called “backflush” or “backpulse”) with the permeate/filtrate or a gas. The backwash method is typically used to eject solids blocking the membrane pores and partly dislodge the cake that may have formed on the membrane surface.
Backwash with pressurized gas has proved a very efficient cleaning method and is now widely used in the field of microfiltration processes. The limitation to this method is the membrane pore size. Backwash of membranes with permeate has no limitations to the pore size, but the backwash efficiency is generally lower than gas backwash and the transmembrane pressure (TMP) recovery not enough to offset the fouling rate. Further means are employed to enhance the backwash efficiency, such as dosing chemicals to the backwash permeate, or in combination with gas scrubbing.
Maruyama et al in Japanese Patent No. JP2031200 discloses a hollow fibre membrane backwashing method. The method involves the following sequence: stop filtration, air-scour membrane, fill the membrane vessel, backwash with permeate under pressurized air and drain the waste. This procedure is repeated to achieve a higher efficiency. Sunaoka et al in a U.S. Pat. No. 5,209,852 describes a process for scrubbing hollow fibre membranes in modules. This process is composed of a two-stage air scrubbing and draining to clean the membranes.
A lot of effort has been made to more effectively lift solids accumulated on the membrane surface and in the pores by optimising the backwash pressure and enhancing the air scrubbing efficiency. Another important step to achieve an efficient cleaning, which has been largely ignored, is the removal of solids that have been exfoliated off the membrane, from the membrane modules. The typical methods presently used are by draining down of the waste or by feed-and-bleed. Feed and bleed involves continual bleeding of waste containing feed out of the filtration system. The outcome is the accumulation of solids within the modules, particularly towards the two ends of a module and the effect becomes more serious if the membranes are densely packed in a module.